1. Field of the Invention
The present invention relates to a semiconductor laser device and a manufacturing method of the same and, more particularly, to a high-output semiconductor laser device and a manufacturing method of the same.
2. Description of the Prior Art
As shown in FIG. 1A, a conventional semiconductor laser is formed by selectively stacking an n-InP cladding layer 22, i-InGaAsP active layer 23, and p-InP cladding layer 24 on an n-InP substrate 21, selectively growing p-InP current blocking layers 25 and n-InP current blocking layers 26 constituting a pnpn thyristor structure on the two sides of the stacked structure, and forming a p-InP over cladding layer 27 and p-InGaAs capping layer 28 on the n-InP current blocking layers 26. In this semiconductor laser, the current blocking layers 25 formed on the two sides of the i-InGaAsP active layer 23 serving as a light emission region effectively confine an injection current in the i-InGaAsP active layer 23. The p-InP current blocking layers 25 play an important role for a high laser output. To enhance the current blocking effect, the current blocking layer must be heavily doped with an impurity.
In this conventional semiconductor laser, the p-InP current blocking layer 25 is heavily doped with an impurity to ensure a high breakdown voltage of the current blocking layer 25. In this case, however, since the p-InP current blocking layer 25 near the i-InGaAsP active layer 23 grows on a high-order plane such as the (311) plane or (211) plane which easily entraps an impurity element, the impurity concentration near the i-InGaAsP active layer 23 increases. That is, in the semiconductor laser, since the migration effect is prompted to grow the p-InP current blocking layer 25, as shown in FIG. 1B, the p-InP current blocking layer 25 near the i-InGaAsP active layer 23 grows on the (111)B plane at the start of growth, and sequentially grows on high-order planes such as the (311) plane and (211) plane owing to the migration effect.
In a general semiconductor laser, light emitted from the i-InGaAsP active layer 23 enters the p-InP current blocking layer 25 to a certain degree. Thus, if the impurity concentration of the p-InP current blocking layer 25 near the i-InGaAsP active layer 23 is high, the free carrier absorption loss in the p-InP current blocking layer 25 increases, failing to attain a high output. In addition, a high impurity concentration near the i-InGaAsP active layer 23 decreases the resistance, and a leakage current from the p-InP cladding layer 24 on the i-InGaAsP active layer 23 to the p-InP current blocking layer 25 tends to increase. This leakage current serves as a gate current to the pnpn current blocking thyristor, and a larger leakage current decreases the breakdown voltage of the blocking layer. Further, the impurity of the p-InP current blocking layer 25 is diffused to the i-InGaAsP active layer 23 to degrade characteristics and reliability due to impurity contamination.
The present invention has been made in consideration of the above situation, and has as its object to provide a high-output semiconductor laser device with high reliability which can reduce the free carrier absorption loss in a current blocking layer and the leakage current, and a manufacturing method of the same.
To achieve the above object, according to the first aspect of the present invention, there is provided a semiconductor laser device comprising an active layer formed on a substrate, and current blocking layers formed on the substrate so as to sandwich the active layer, wherein each current blocking layer has a low impurity concentration at a portion near the active layer and a high impurity concentration at a portion apart from the active layer.
In the first aspect, the portion of the current blocking layer near the active layer is a region 1 to 2 xcexcm and preferably not more than 1 xcexcm apart from an end of the active layer.
In the first aspect, the current blocking layer has an impurity concentration of 3 to 5xc3x971017 cmxe2x88x923 at the portion near the active layer, and an impurity concentration of 7 to 10xc3x971017 cmxe2x88x923 at the portion apart from the active layer.
To achieve the above object, according to the second aspect of the present invention, there is provided a manufacturing method of a semiconductor laser device, comprising the steps of forming an active layer on a substrate, and selectively forming current blocking layers on the substrate by metal organic vapor phase epitaxy so as to sandwich the active layer, wherein a growth condition along the metal organic vapor phase epitaxy of each current blocking layer is determined such that an impurity concentration is low at a portion near the active layer and high at a portion apart from the active layer in the step of forming the current blocking layers.
In the second aspect, the first growth condition along the metal organic vapor phase epitaxy of the current blocking layer is to set a growth temperature in an initial stage of metal organic vapor phase epitaxy for the current blocking layer to be lower than a subsequent growth temperature. The second growth condition is to set a growth pressure in the initial stage of metal organic vapor phase epitaxy for the current blocking layer to be higher than a subsequent growth pressure. The third growth condition is to set a growth rate in the initial stage of metal organic vapor phase epitaxy for the current blocking layer to be higher than a subsequent growth rate. The fourth growth condition is to continuously supply a source gas of Group V in the initial stage of metal organic vapor phase epitaxy for the current blocking layer and to subsequently intermittently supply the source gas of Group V. The fifth growth condition is to increase a ratio of a source gas of Group III to a source gas of Group V in the initial stage of metal organic vapor phase epitaxy for the current blocking layer and to subsequently decrease the ratio.
As is apparent from these aspects, in the semiconductor laser device of the present invention, the impurity concentration of the current blocking layer near the active layer can be decreased without decreasing the breakdown voltage of the current blocking layer. Hence, the free carrier absorption loss in the current blocking layer can be reduced to obtain a high optical output and high reliability.
By adopting the manufacturing method of the semiconductor laser device according to the present invention, for example, the migration effect of a source species is suppressed at a portion near an i-InGaAsP active layer in the initial growth stage during formation (growth) of a p-InP current blocking layer. The p-InP current blocking layer keeps growing on the (111)B plane. To the contrary, the migration effect of the source species is prompted by changing crystal growth conditions at a portion apart from the active layer in the latter growth stage, and the current blocking layer grows on high-order planes such as the (311) plane and (211) plane. When the crystal growth plane is a high-order plane such as the (311) plane or (211) plane, a dangling bond is more readily formed during growth, and a larger amount of impurity is entrapped, compared to the (111)B plane having a very low growth rate. Therefore, the p-InP current blocking layer has a low impurity concentration near the active layer and a high impurity concentration at a portion apart from the active layer.
As described above, according to the present invention, since the impurity concentration of the current blocking layer near the active layer is relatively low, the free carrier absorption loss during propagation of light emitted by the active layer can be reduced. In addition, since the impurity concentration of the current blocking layer apart from the active layer is relatively high, the breakdown voltage of the current blocking layer can be kept high. Since the impurity concentration of the current blocking layer near the active layer is relatively low, the resistance near the active layer is high, and the leakage current flowing from an over cladding layer immediately above the active layer to the current blocking layer via a cladding layer can be reduced to increase the breakdown voltage of the current blocking layer. This can realize a high efficiency and high breakdown voltage, and can provide a high-optical-output semiconductor laser device. Moreover, since the impurity concentration of the current blocking layer in contact with the active layer is relatively low, diffusion of the impurity to the active layer can be suppressed. A semiconductor laser device with high reliability can be obtained.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.